The present invention relates to a signal evaluation circuit for a motion detector which contains a sensor and whose sensor signal contains a relatively large direct current portion and a small alternating current portion, having means for filtering out the direct current portion, having an analogue-to-digital converter and having an amplifier for the alternating current portion of the sensor signal.
The sensor signal of motion detectors of this type is composed of an highly erratic and temperature-dependent direct current component and of an alternating current portion. The direct current portion which makes no contribution to the useful signal cannot be anticipated and is unstable for a relatively long period of time, and the alternating current portion which delivers the useful signal in order to trigger the alarm is at a level of approximately one per thousand of the direct current portion and therefore has to be amplified to a correspondingly intensive degree. Generally the signal evaluation circuit contains a series of capacitors which act as high-pass filters and gradually filter out the direct current portion. The remaining alternating current signal is then digitized and amplified. Owing to the low useful frequencies, the filtering process requires large coupling-electrolyte capacitors which are not only expensive and problematic from the electrical point of view but which also cannot be integrated and thus render it impossible to construct the evaluation circuit as an integrated circuit (IC) which would be desirable for reasons of cost.